Ensure PSR-7 body->read() is called with an int argument per spec

The spec requires that the length argument of the body read method be
an integer, but Slim's implementation causes Content-Length to be
retrieved as a string due to its use of $response->getHeaderLine. This
causes compatibility issues with PSR-7 implementations that use strict
typing, namely zendframework/zend-dictoros 2.0+.

Slim/App.php

@@ -461,7 +461,7 @@ public function respond(ResponseInterface $response)
             if (isset($contentLength)) {
                 $amountToRead = $contentLength;
                 while ($amountToRead > 0 && !$body->eof()) {
-                    $data = $body->read(min($chunkSize, $amountToRead));
+                    $data = $body->read(min((int)$chunkSize, (int)$amountToRead));
                     echo $data;
 
                     $amountToRead -= strlen($data);